Obadare O. Awoleke

Preliminary Study of the Carbon Sequestration and Enhanced Coal Bed Methane Production Potential of Subbituminous to High-Volatile Bituminous Coals of the Healy Creek Formation, Nenana Basin, Interior Alaska

Naturally fractured, unmineable coal seam reservoirs are attractive targets for geological sequestration of carbon dioxide (CO2) because of their high CO2 adsorption capacity and possible cost offsets from enhanced coal bed methane production. In this study, we have investigated the CO2 sequestration and coal bed methane (CH4) production potential of the subbituminous to high-volatile C bituminous Healy Creek Formation coals through preliminary sensitivity analyses, experimental design methods, and fluid flow simulations. The sensitivity analyses indicate that the total volumes of CO2 sequestered and CH4 produced from the Healy Creek coals are mostly sensitive to bottom-hole injection pressure, coal matrix porosity, fracture porosity, fracture permeability, coal compressibility, and coal volumetric strain. The results of the Plackett–Burman experimental design were used to further generate proxy models for probabilistic reservoir forecasts. The probabilistic estimates for the mature, subbituminous to high-volatile C bituminous Healy Creek coals in the entire Nenana Basin indicate that it is possible to sequester between 0.41 trillion cubic feet (TCF) (P10) and 0.05 TCF (P90) of CO2 while producing between 0.36 TCF (P10) and 0.05 TCF (P90) of CH4 at the end of 44-year forecast. Fluid flow scenarios show that CO2 sequestration through a primary reservoir depletion method is the most effective way to inject CO2 in the coals of the Nenana Basin. Including a horizontal well instead of the vertical well resulted in relatively high average gas production rates and subsequent total cumulative gas production. The CO2 buoyancy scenario suggests that the effect of CO2 buoyancy and the nature of caprock should be considered in identifying potential geologic sites for CO2 sequestration.

In situ stress variations associated with regional changes in tectonic setting, northeastern Brooks Range and eastern North Slope of Alaska

ABSTRACT The northeastern Brooks Range of northern Alaska is an active, north-directed fold-and-thrust belt that is advancing on the Barrow arch and the north-facing passive margin of the Arctic Basin. Density logs, leak-off tests, and mud-weight profiles from 57 wells from the northeastern North Slope were used to determine the magnitude of the present-day in situ stresses and document significant regional lateral and vertical variations in relative stress magnitude. Preliminary analysis of the in situ stress magnitudes indicates two distinct stress regimes across this region of Alaska. Areas adjacent to the eastern Barrow arch exhibit both strike-slip and normal stress regimes. This in situ stress regime is consistent with fault patterns in the subsurface and with north–south extension along the Barrow arch and the northern Alaska margin. To the south in and near the northeastern Brooks Range thrust front, in situ stress magnitudes indicate that an active thrust-fault regime is present at depths up to 6000 ft (1829 m). This is consistent with the fold-and-thrust structures in surface exposures and in the subsurface. However, at depths greater than 6000 ft (1829 m), the relative in situ stress magnitudes indicate a change to a strike-slip regime. This observation is consistent with the few earthquake focal mechanisms in the area and suggests deep north-northeast–oriented strike-slip faults may underlie the western margin of the northeastern Brooks Range.